I would wonder about making it in two steps since each element has different oven times and temps. Make the Scalloped Potato dish, cool and reserve. When you want to serve, lay the puff pastry on top of room temp scalloped potato (with egg wash), and bake until crispy brown. Serve, accept applause graciously.

I think bigpan is right regarding after the puff pastry . Martha (and please don't take this as a criticism - I just get to use one of my mother's favorite phrases), this could be called "gilding the lily" - potatoes, cream, probably cheese AND puff pastry. Very decadent.

Something I like on top of scalloped potatoes is a sprinkling of panko or regular (unseasoned) bread crumbs mixed with some chopped parsley and a little grated swiss or gruyere, dotted with tiny bits of butter. You can bake the potato part of the dish first and if you freeze or refrigerate that, bring it to room temp to defrost or get the chill off, sprinkle the bread crumb/cheese mixture on top and warm it up in the oven at 350 for 30 minutes. (I like my food very hot, so I'd probably leave it in the oven for even a little longer.) You might need to put it under the broiler for just a minute or two to brown the top nicely without over-cooking the potatoes.